diff --git a/lib/Tools/components b/lib/Tools/components
--- a/lib/Tools/components
+++ b/lib/Tools/components
@@ -1,175 +1,157 @@
 #!/usr/bin/env bash
 #
 # Author: Makarius
 #
 # DESCRIPTION: resolve Isabelle components
 
 
 ## diagnostics
 
 PRG="$(basename "$0")"
 
 function usage()
 {
   echo
   echo "Usage: isabelle $PRG [OPTIONS] [COMPONENTS ...]"
   echo
   echo "  Options are:"
   echo "    -I           init user settings"
   echo "    -R URL       component repository (default \$ISABELLE_COMPONENT_REPOSITORY)"
   echo "    -a           resolve all missing components"
   echo "    -l           list status"
   echo "    -u DIR       update \$ISABELLE_HOME_USER/etc/components: add directory"
   echo "    -x DIR       update \$ISABELLE_HOME_USER/etc/components: remove directory"
   echo
   echo "  Resolve Isabelle components via download and installation: given COMPONENTS"
   echo "  are identified via base name. Further operations manage etc/settings and"
   echo "  etc/components in \$ISABELLE_HOME_USER."
   echo
   echo "  ISABELLE_COMPONENT_REPOSITORY=\"$ISABELLE_COMPONENT_REPOSITORY\""
   echo "  ISABELLE_HOME_USER=\"$ISABELLE_HOME_USER\""
   echo
   exit 1
 }
 
 function fail()
 {
   echo "$1" >&2
   exit 2
 }
 
 
 ## process command line
 
 #options
 
 INIT_SETTINGS=""
 COMPONENT_REPOSITORY="$ISABELLE_COMPONENT_REPOSITORY"
 ALL_MISSING=""
 LIST_ONLY=""
 declare -a UPDATE_COMPONENTS=()
 
 while getopts "IR:alu:x:" OPT
 do
   case "$OPT" in
     I)
       INIT_SETTINGS="true"
       ;;
     R)
       COMPONENT_REPOSITORY="$OPTARG"
       ;;
     a)
       ALL_MISSING="true"
       ;;
     l)
       LIST_ONLY="true"
       ;;
     u)
       UPDATE_COMPONENTS["${#UPDATE_COMPONENTS[@]}"]="+$OPTARG"
       ;;
     x)
       UPDATE_COMPONENTS["${#UPDATE_COMPONENTS[@]}"]="-$OPTARG"
       ;;
     \?)
       usage
       ;;
   esac
 done
 
 shift $(($OPTIND - 1))
 
 
 # args
 
 [ "$#" -eq 0 -a -z "$INIT_SETTINGS" -a -z "$ALL_MISSING" -a -z "$LIST_ONLY" -a "${#UPDATE_COMPONENTS[@]}" -eq 0 ] && usage
 
 if [ -z "$ALL_MISSING" ]; then
   splitarray ":" "$@"
 else
   splitarray ":" "$ISABELLE_COMPONENTS_MISSING" "$@"
 fi
 declare -a SELECTED_COMPONENTS=("${SPLITARRAY[@]}")
 
 
 ## main
 
 splitarray ":" "$ISABELLE_COMPONENTS"; declare -a AVAILABLE_COMPONENTS=("${SPLITARRAY[@]}")
 splitarray ":" "$ISABELLE_COMPONENTS_MISSING"; declare -a MISSING_COMPONENTS=("${SPLITARRAY[@]}")
 
 if [ -n "$INIT_SETTINGS" ]; then
   SETTINGS="$ISABELLE_HOME_USER/etc/settings"
   SETTINGS_CONTENT='init_components "${ISABELLE_COMPONENTS_BASE:-$USER_HOME/.isabelle/contrib}" "$ISABELLE_HOME/Admin/components/main"'
   if [ ! -e "$SETTINGS" ]; then
     echo "Initializing \"$SETTINGS\""
     mkdir -p "$(dirname "$SETTINGS")"
     {
       echo "#-*- shell-script -*- :mode=shellscript:"
       echo
       echo "$SETTINGS_CONTENT"
     } > "$SETTINGS"
   elif grep "init_components.*components/main" "$SETTINGS" >/dev/null 2>/dev/null
   then
     :
   else
     echo "User settings file already exists!"
     echo
     echo "Edit \"$SETTINGS\" manually"
     echo "and add the following line near its start:"
     echo
     echo "  $SETTINGS_CONTENT"
     echo
   fi
 elif [ -n "$LIST_ONLY" ]; then
   echo
   echo "Available components:"
   for NAME in "${AVAILABLE_COMPONENTS[@]}"; do echo "  $NAME"; done
   echo
   echo "Missing components:"
   for NAME in "${MISSING_COMPONENTS[@]}"; do echo "  $NAME"; done
 elif [ "${#UPDATE_COMPONENTS[@]}" -ne 0 ]; then
   isabelle scala_build || exit $?
   exec isabelle java isabelle.Components "${UPDATE_COMPONENTS[@]}"
 else
+  source "$ISABELLE_HOME/lib/scripts/download_file"
   for NAME in "${SELECTED_COMPONENTS[@]}"
   do
     BASE_NAME="$(basename "$NAME")"
     FULL_NAME=""
     for X in "${AVAILABLE_COMPONENTS[@]}" "${MISSING_COMPONENTS[@]}"
     do
       [ -z "$FULL_NAME" -a "$BASE_NAME" = "$(basename "$X")" ] && FULL_NAME="$X"
     done
     if [ -z "$FULL_NAME" ]; then
       echo "Ignoring irrelevant component \"$NAME\""
     elif [ -d "$FULL_NAME" ]; then
       echo "Skipping existing component \"$FULL_NAME\""
     else
       if [ ! -e "${FULL_NAME}.tar.gz" ]; then
-        REMOTE="$COMPONENT_REPOSITORY/${BASE_NAME}.tar.gz"
-        type -p curl > /dev/null || fail "Cannot download files: missing curl"
-        echo "Getting \"$REMOTE\""
-        mkdir -p "$(dirname "$FULL_NAME")"
-
-        CURL_OPTIONS="--fail --silent --location"
-        if [ "$ISABELLE_PLATFORM_FAMILY" = "macos" ]; then
-          case $(sw_vers -productVersion) in
-            10.*)
-              CURL_OPTIONS="$CURL_OPTIONS --insecure"
-              ;;
-          esac
-        fi
-        if curl $CURL_OPTIONS "$REMOTE" > "${FULL_NAME}.tar.gz.part"
-        then
-          mv -f "${FULL_NAME}.tar.gz.part" "${FULL_NAME}.tar.gz"
-        else
-          rm -f "${FULL_NAME}.tar.gz.part"
-          fail "Failed to download \"$REMOTE\""
-        fi
+        download_file "$COMPONENT_REPOSITORY/${BASE_NAME}.tar.gz" "${FULL_NAME}.tar.gz" || exit $?
       fi
       if [ -e "${FULL_NAME}.tar.gz" ]; then
         echo "Unpacking \"${FULL_NAME}.tar.gz\""
         tar -C "$(dirname "$FULL_NAME")" -x -f "${FULL_NAME}.tar.gz" || exit 2
       fi
     fi
   done
 fi
 
diff --git a/lib/scripts/download_file b/lib/scripts/download_file
new file mode 100644
--- /dev/null
+++ b/lib/scripts/download_file
@@ -0,0 +1,40 @@
+# -*- shell-script -*- :mode=shellscript:
+#
+# Bash function to download file via "curl".
+
+function download_file ()
+{
+  [ "$#" -ne 2 ] && {
+    echo "Bad arguments for download_file" >&2
+    return 2
+  }
+  local REMOTE="$1"
+  local LOCAL="$2"
+
+  type -p curl > /dev/null || {
+    echo "Require \"curl\" to download files" >&2
+    return 2
+  }
+
+  local CURL_OPTIONS="--fail --silent --location"
+  if [ "$(uname -s)" = "Darwin" ]
+  then
+    case $(sw_vers -productVersion) in
+      10.*)
+        CURL_OPTIONS="$CURL_OPTIONS --insecure"
+        ;;
+    esac
+  fi
+
+  echo "Getting \"$REMOTE\""
+  mkdir -p "$(dirname "$LOCAL")"
+
+  if curl $CURL_OPTIONS "$REMOTE" > "${LOCAL}.part"
+  then
+    mv -f "${LOCAL}.part" "$LOCAL"
+  else
+    rm -f "${LOCAL}.part"
+    echo "Failed to download \"$REMOTE\"" >&2
+    return 2
+  fi
+}